<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <!-- 
        我们可以把不同宽度下的样式写在不同的css文件里, 然后引入 
        利用link中的media属性，语法和css里的@media一样的, 和css里一样要注意引入顺序, 
    -->
    <link rel="stylesheet" href="./style320.css" media="screen and (min-width: 320px)">
    <link rel="stylesheet" href="./style640.css" media="screen and (min-width: 640px)">
</head>
<body>
    <div>1</div>
    <div>2</div>
</body>
</html>